UIToolkit UIProgressBar methods changed

I'm developing a 3D game with Unity and in order to build the button I use UIToolKit. I saw a tutorial which showed how to create a UIProgressBar with UIToolKit, the problem is that it was from 2 years ago and the methods to create a UIProgressbar have changed. Before you could set both progress bar and the progress bar border images, but now you can only set the progress bar image. How can I set the border? Have I got to set a sprite only for the border? Does someone know something about it?